Hello, i have a Dell Optiplex 740 AMD-ATHLON 64bit 2.4GHZ core 2 Duo 5000+

Recenlty purchased i am having a serious problem, bought it off at a seconf hand shop for cheap and i've installed Vista Ultimate 64x on it, i was told the fan was loud but when turning it on the fan runs at FULL SPEED and doesnt slow down even when the PC is not running any serious applciations!

I've tried looking every where to see if there's a fix for it, i recently updated the BIOS hoping to resolve it but no Joy....

I even pressed F2 at start up to see if i can control the fan speed there but dells dont seem to support that feature.

I'm really stuck for things to look for, maybe the heatsink is not seated correctly or the fan's bearings have worn so it cant slow down, who knows....

it runs so fast it makes my hoover look bad and it completely freezes my small room and its basically unbare-able

Please can anyone help, is there a program i can install that can give me a temporay fix or solution? any suggestions a knowledgable advice will be appreciated.
Thanks